Shotguns, Spears, and the new Psi school are all great additions, and I'd recommend trying each of them (especially spears). The artwork is beautiful throughout, and exploration is really cranked up to ten. There is the occasional boring or repetitive location (Nexus of Technology and the Horticultural Center), but these are also contrasted with some of the best dungeons in the main game (Joint Security Headquarters, Mutie Island/West Storage Depot, Leumerian Manufacturing Complex, Pirate Base, Abyssal Station Zero, Natives, etc). Expedition is a little wordier, but I consider this to be a good thing. There are a ton of memorable characters, and the overall dichotomy of the expedition versus the pirates is really cool, so I'd recommend a playthrough where you help each side once to see the different content.Is Expedition any good? Is it worth starting another build? How many hours for a completionist expansion playthrough?
The encounter design isn't as good as the main game, but it's not nearly as bad as some people make it out to be. The crabs coming out of the ground and the strongmen suddenly turning on can be a bit annoying, but it also forces you to approach each encounter in a specific way to best handle a tough fight. Locusts are aids, but you really only have to fight them in one area and can avoid them the rest of the game after that. Just bring some MK2/3 grenades and bomb them to hell. If you have sneak, sneak up and TNT the hives.
TL;DR: Play it.